In particle therapy, the scanning method has been gaining widespread acceptance. The scanning method involves dividing a target into minute regions (called spots hereunder) and irradiating each spot with a small-diameter beam. When a spot has been irradiated with a predetermined dose, the irradiation with the beam is stopped and the beam is moved to scan the next spot. Where the beam is to be moved for scanning in a direction (lateral direction) perpendicular to the beam advancing direction (depth direction), a scanning magnet is used. When all spots at a given depth have been irradiated with the predetermined dose, the beam is moved for scanning in the depth direction. Where the beam is to be moved for scanning in the depth direction, the energy of the beam is changed by an accelerator or by a range shifter. Eventually, all spots (i.e., the entire target) are irradiated with a uniform amount of dose.
The beam for each spot has a two-dimensional Gaussian distribution in the lateral direction. On the isocenter plane, 1σ is about 3 to 20 mm; the value is smaller the higher the beam energy. A low-energy beam has a large angular divergence per unit distance due to multiple coulomb scattering. The beam increases in diameter as it passes through an irradiation nozzle.
Thus the low-energy beam is used to form the dose distribution for the target located at a shallow position (called the short-range region) from the surface of the irradiated body. This tends to increase penumbrae. Outside the target or in a region near the boundary between the target and the normal tissue, penumbrae represent the lateral distance over which the irradiation dose drops from 80% to 20% and have positive correlation with the beam diameter. It is assumed here that the irradiation dose near the target center is 100%. The smaller the penumbrae, the more accurate the dose distribution is in conformance with the shape of the target.
Given that problem, Non-Patent Document 1 proposes a technique that involves setting up an energy absorber upstream of the irradiated body. According to this technique, a high-energy beam is emitted to the target in the short-range region and is reduced in energy by the energy absorber immediately before entering the irradiated body. Because the drift distance of the beam in the low-energy state is suppressed, the beam diameter can be reduced and the penumbrae improved. Meanwhile, Non-Patent Document 2 proposes a technique involving the use of a collimator to block the beam that is incident outside the target, thereby improving the penumbrae.